When a machine tool mounted with a numerical control apparatus is caused to perform machining of a three-dimensional shape, the machining could be performed according to a machining program in which free-form surfaces are approximated in a continuous plurality of command paths. The machining program could be manually created if a shape to be machined is a simple shape. However, in the case of a three-dimensional shape including a free-form surface, in general, the three-dimensional shape is created by a CAM (Computer Aided Manufacturing) on an external apparatus different from the numerical control apparatus.
When a machining program is created using the CAM, to represent a free-form surface as accurately as possible, it is necessary to reduce the length of one command path (hereinafter referred to as path length). However, the number of command paths that can be processed in a fixed time is limited by a data processing ability of the numerical control apparatus. Therefore, when the path length is reduced, a distance that the tool can move in the fixed time, i.e., feeding speed of the tool is limited.
Therefore, in the numerical control apparatus in the past, the path length is increased by replacing a continuous plurality of command paths in the same straight line section with one command path (compressing the command paths) and high-speed machining is performed (e.g., Patent Document 1).